This invention relates to fluorescent lamps which are adapted as direct replacements for existing incandescent lamps. More specifically, this invention concerns substantially globular fluorescent lamps wherein ionization is induced by a transformer which is positioned substantially in the center of, but external to, the lamp envelope.
The incandescent lamp is the primary luminary for household and residental lighting. This lamp generally includes an incandescent filament within a predetermined nonoxidizing atmosphere which is contained with a teardrop shaped envelope and mounted, for example, within an Edison type base which is screwed into a permanent fixture or into a movable socket.
Despite their widespread use, incandescent lamps are relatively inefficient, producing only 15-17 lumens per watt of input power and have relatively short, unpredictable service lives. Fluorescent lamps, which have efficiencies as high as 80 lumens per watt, provide an attractive alternative to incandescent lighting. Conventional fluorescent lamps, however, require a long tubular envelope which, together with the need for auxiliary ballasting equipment, has somewhat limited their acceptance in the home lighting market. Increased residential use of fluorescent illumination, with attendant savings of energy, can be achieved from the development of fluorescent lamps which are directly compatible with existing sockets and incandescent lamp fixtures.
The electric lamp technology has long sought electric discharge devices which produce visible light for general illumination purposes without the utilization of electrodes as the footpoints of a glow or arc discharge. Although the concept of electrodeless discharge lamps is very old, such lamps have always included the concept of coupling electrical energy into an hermetically sealed gas-containing envelope by means of a ferromagnetic or air core transformer to avoid the use of electrodes. Such devices have never proved practical or commercially feasible, because it has been impossible to achieve any reasonable efficiency of light emission due to the utilization of iron or air core transformers because of core losses, among other factors.
It has been proposed in the prior art to excite electrodeless gaseous discharge lamps using electromagnetic induction to transfer electric energy into the discharge vessel. Experiments along this line disclose that heretofore such means have been highly impracticable. If an air core transformer is utilized, the inefficiency of the coupling procedure required to achieve a reasonable power into the gaseous discharge results in a loss of power by radiation which is prohibitive, and which may be dangerous. Accordingly, such devices have never been successfully operated for useful periods at any reasonable efficiency.
Another alternative that has been proposed in the prior art is the utilization of an iron or ferromagnetic core. Such prior art cores, however, may be utilized only at very low frequencies in order that eddy current heating of the iron does not cause core failure. Utilizing alternating current, it is exceedingly difficult to operate an iron core transformer for the purpose of transferring energy of this nature at frequencies in excess of 5 or 10 kilohertz. Based upon experimental and calculated results obtained in this laboratory, it has been determined that for an iron core transformer operating at 50 kHz, core power losses are in the range of approximately 80 to 90 percent. Accordingly, from the foregoing it may readily be appreciated that air core and iron core transformers are, from a practical point of view, inoperative at the high radio frequency levels that are necessary for efficient operation of gaseous discharge lamps in accord with this invention.
In prior U.S. Pat. Nos. 3,500,118 and 3,521,120 there are disclosed fluorescent lamps which utilize a magnetically induced radio frequency electric field to ionize a gaseous radiating medium. The elimination of discharge electrodes within these lamp envelopes substantially increases their life and allows lamp shapes which are more compatible with home lighting needs.
U.S. Pat. No. 3,500,118, issued Mar. 10, 1970, describes an improved electrodeless fluorescent lamp having a radio frequency power supply. This lamp, while quite useful, is bulky, comprising a large tubular discharge ring, several ferrite cores, and a remotely mounted power supply which makes it unsuitable for use in many industrial and residential applications.
U.S. Pat. No. 3,521,120 issued July 21, 1970, describes a more compact lamp configuration. However, this lamp maintains a high frequency magnetic field in the air surrounding the envelope and thus constitutes a source of electromagnetic radiation and interference.
Breifly stated, concurrently filed U.S. patent applications Ser. Nos. 642,142 and 642,148 disclose electrodeless fluorescent lamps which are adapted as replacements for globular incandescent lamps. These lamps comprise a substantially globular, light-transmissive envelope which is phosphor coated and filled with an ionizable gas. A radio frequency, closed loop core, transformer lying at least partially within that envelope is used to induce a radio frequency electric field which ionizes the gas. These lamps have physical dimensions and electrical characteristics compatible with existing incandescent lighting equipment. Both of the above-mentioned lamps, however, require seals between the light transmissive envelope and either metal or magnetic core components. The bulk of the heat produced by losses within the lamps' envelope must be transmitted through these metal or core components. This heat transfer process produces differential thermal expansion between the sealing materials with resultant stresses in the seal structure. Because of these stresses, the seals represent a site for potential lamp failure and a substantial portion of the lamps' production cost.